How can I select the faces that haven't been UV mapped yet in a model?
Selection in the UV origin

Each UV map always contains the UV island relative to each face: if they hasn't been unwrapped yet the way I imagine, you'll find them collapsed in a point in the origin of the UV space (0;0).



In the example above only the upper part of the head has been unwrapped. The black dot in the center of the 2D cursor are the island of all the other faces.



You just have to take advantage of any of the selection tool (e.g. Box-Selection) while keeping UV and edit mode mesh selection in sync.